The present invention relates to a vehicle on-board diagnosing system and more particularly to a system which includes a group of units for diagnosing a plurality of systems, respectively, and a display operatively coupled with the units so as to inform the results of diagnosis.
There is known a vehicle on-board diagnosing system as illustrated in FIG. 3. 21 designates a display unit which informs the result of diagnosis. With a harness 6, the display unit 21 is connected to the units 2, 3, 4, and 5 for diagnosing systems, respectively. The systems represent, for example, an anti-skid control system, a transmission control system, an engine control system, and etc. The display unit 21 is connected to each of the units 2, 3, 4, and 5 via two signal lines, one of which is a data line for delivering data indicating result of diagnosis made at the corresponding unit and the other of which is an activating mode instruction line. Also included by the display unit 21 are a data input processing circuit 31 and an activating switch 32. With the activating switch 32, the data processing circuit 31 selects one of the units 2, 3, 4, and 5. The display unit 21 further includes a data discrimating circuit 33 which discriminates a system being under diagnosis and an activating mode based on the data coming out of the data input processing circuit 31. An output signal of the discriminating circuit 33 is supplied to a display control circuit 34. The display control circuit 34 is operatively connected with a display 16 to control the latter. A display check switch 35 is operatively connected with the display panel 16. Telecommunication self-check, trouble check, input/output system check are examples of the activating mode.
Since, according to this known diagnosing system, the units 2, 3, 4, and 5 are connected to the data input processing circuit 31 of the display unit 21 via different signal lines, respectively, the display unit 21 must be designed taking into account the number at the control units so that the display 21 cannot cope with the situation where a new control unit is to be added. Thus, it is necessary to prepare different displays for different types and grade of, vehicles. Furthermore, since the control units 2, 3, 4, and 5 arranged at different locations of a vehicle are directly connected to the display 21 mounted within a cabin of the vehicle via different signal lines, the total length of the signal lines increase considerably if the number of the signal lines increases, presenting a problem on reliability and economy.